Pedal-assisted bikes are bicycles with an auxiliary electric motor having the following specifications:                maximum continuous power rating of the electric motor: 0.25 kW;        motor supply gradually reduced and interrupted when 25 km/h is achieved;        motor supply interrupted before 25 km/h if the cyclist stops pedalling.        
The electric motor is controlled by means of a processing and control unit which controls the power supply according to the pedal force of the user, balancing the engagement of the transmission gradually from when pedalling starts, to make this smooth and regular.
For this purpose, pedal-assisted bikes have a force sensor in correspondence to the pedals or a speed sensor positioned in the pedal crank or both.
In traditional pedal-assisted bikes, the electric motor is housed directly in the hub of the motorised wheel and is powered by a battery pack which, instead, is fitted on the bicycle frame.
Considering the need to equip the bike with all the aforementioned components designed to perform various different functions, the traditional pedal-assisted systems are usually conceived, right from the design stage, to be fitted on a specific bike model.
Only rarely in fact is it possible to develop a kit able to adapt to several bike models, taking into account the considerable differences between the bicycles on the market, in particular their frames which, of course, have shapes and dimensions so different as to prevent a practical and complete standardisation of pedal-assisted systems.
It must also be underlined that the fitting of a kit for pedal-assisted bikes during the after-sales stage, even if this were possible, usually calls for the performance of inconvenient and not very practical operations concerning the adjustment and the fitting of the various components to the frame which more often than not cannot be performed by the end user and requires the intervention of an expert technician.
To partially overcome the aforementioned drawbacks, the integration is known in the motorised wheel of not only the electrical drive motor but also of the battery pack, as described for example in the patent document WO 2010/091323.
This constructive solution however is not without drawbacks either.
In this respect, it should first of all be noticed that the integration of the battery pack in the motorised wheel only in part simplifies pedal-assisted system fitting and removal operations because it in any case remains necessary to install force and/or speed sensors on the bike pedal crank, as well as all other components.
To this must be added that, quite apart from where the battery pack is positioned either on the wheel or on the frame, the motorised wheels for pedal-assisted bikes can not always be combined in a practical and easy way with the traditional mechanical gears usually used to reduce or multiply the motion transmission ratio from the pedal crank.
The fact that the hub of the motorised wheel is almost completely occupied by the electric motor in fact determines the fact that this wheel cannot be combined with gear change devices inside the hub, such as, e.g., the Rohloff gear change but can only be combined with external devices, such as traditional derailleur gears, which nevertheless are not standardised and vary according to the bike model.